In the prior art, the electronic power steering apparatus for applying the steering assist force to the steering mechanism of the vehicle by driving the electronic motor in response to the steering torque that the driver applies to the steering wheel (handle) is employed. As the electronic motor of the electronic power steering apparatus, the brush motor is employed widely in the prior art. From the aspects of an improvement in reliability and durability, a reduction in inertia, etc., the brushless motor is also employed nowadays.
In order to control a torque produced by the motor, commonly the motor controller detects an electric current flowing through the motor, and then applies the PI control (proportional integral control) based on a difference between an electric current that is to be supplied to the motor and an electric current that is detected. This motor is the three-phase brushless motor, for example. In order to detect the electric current in two phases or more, two or three current sensors are provided to the motor controller. Also, since a rotation angle of the motor is employed in the d-q axis/three-phase convention, various operations, etc., whose details are described later, the sensor such as the resolver, the Hall IC, or the like is provided to sense the rotation angle of the motor.
In connection with the invention of this application, in JP-A-9-331693 and JP-A-2006-56473, the configuration for estimating a rotation angle of the motor not to use the rotation angle sensor or the angular velocity sensor for the motor is disclosed. Also, in JP-A-2007-118823 and JP-A-2008-37399, the configuration for estimating a rotation angle of the motor when a failure is caused in the motor rotation angle detecting section is disclosed.